Ongoing Contest(Scores)

[Balsa Man]

I am not a paid sponsor of that or any site. I just answered the question asked.

Can it handle built up beams, yes, with a fair amount of extra work That is beyond my ability and need.
Yes, it does cover axial and bending stresses.
You can even put side loads on it.

Is the same size and density the same strength--certainly not, as most balsa users know. It will handle any wt of wood that you choose. If you do not like the values for any of the variables, you can change those as desired.

All of that is above nearly all students and most coaches.

It is written by a retired LSU engineering professor and an active ME.

If anyone does not like it, don't buy it.

That said, it has worked very well for me on bridges and towers.

Wow, I never cease to be amazed how easy it is for e-mail/posts to be misinterpreted/misunderstood, in terms of intent/tone.

From your reply, it sounds like you read some things into what I posted that were far from my intent/thoughts, and I am reading a sense of…..defensiveness in your reply that may or may not be real.

I had/stated a couple specific questions for you- as a proficient user of the program you mentioned– things I was curious about. And as I often do, put out for…..the broader audience some general context info on a subject- things to think about/be aware of, add to your knowledge base. Sometimes, as in this case, I put out what I think is new info into our collective knowledge base.

For anyone attempting to work up design performance and weight – theoretical, before any building, by whatever tool, the “real’ values for “E” are important. That, in my mind was the…..thrust of my post this time- providing access to new info on this I’d found. That’s all.

The whole ‘paid sponsor’/pushing buying this tool thing, don’t know where that’s coming from; certainly not an element of anything I said/thought.
As I said at the outset, a very, very cool tool.
Hope the E data is useful- to you, and anyone else into design enough to understand itss importance.
No offense intended; hopefully none taken.

[retired1]

Sorry if I took it the wrong way. The world had just taken a major league dump on me and I was more than a bit upset.
I do not consider myself a proficient user of this program. It has capabilities far beyond my understanding. Your E comment is a bit beyond my understanding, since I have never used it and do not know where to find it reduced to a simple number. I just have the kids be careful in their choice of wood flexing.
Will have to re read it several times.

The bit about my not being a paid shill is because I did not want anyone to think that I had a vested interest in it. It is the first time that I have even said that I used it, hoarding it a bit like your accelerator. I am a bit hesitant to make recommendations on kits. I have been more vocal recently than ever before.

It is a PRE engineering program and obviously can not include everything, especially at the price it is offered.
I think that it is a useful program for any that want to invest the time in semi mastering it. For a student that is going to be in SO for 3 or more years or really speaks engineering, it would be a good investment, from my use of it.

Please note: the program is tied to one computer. If you change computers you need to get a new number and the old one cancelled. Trust me, it knows the difference

I am playing around with side loading now.. It really drops the max load. The question is, how much to expect during the competition. We pretty much ignored it on the towers, but they had a large base, absolute vertical and careful loading.

[Balsa Man]

No problem. The written word is easy to take various wrong ways...
Proficient user? Certainly more so than me- all I have to go on is what's on their web page. Limitations, sure- the key to using such a tool is understanding where the limitations lie, and when they can and can't be worked around. At the price, a tool worth considering, for sure.

Briefly on "E." (and E & I together).
In terms of being able to understand (design/model up front) what "lumber" it will take to carry a given axial load, in terms of column/buckling behavior, this is critical, and pretty fundamental. Special cases can get a lot more complicated, but simplified column behavior is pretty straightforward. At a given length, it's all about E and I.
Yes, in compression member(s) of a boom, there are other factors that will play in, and further reduce buckling strength from what you can simply calculate from Euler's equation- torque, and being less than straight are probably the biggest. The anisotropicity (a mouthful) of wood is the other major one- Euler's equation assumes an isotropic material.
When you say, "Your E comment is a bit beyond my understanding, since I have never used it and do not know where to find it reduced to a simple number"; now (with the link I posted) you do; it is as I briefly stated NOT A simple number- it varies with the density, and it varies within a given density. As I noted, and you've seen in the program you're talking about, there is a value box for E; it looks, but i can't tell from just looking at their website, that that value is driven from their "classification" table of varying densities - like you select a classification, or plug in a density, and it retrieves/uses a value for E to calculate buckling strength of a member under compression. What i don't know is whether you can direct plug in an E value (let me know when you check that out). if you can, you are good to go; if it's locked in to its loaded set of E-values.....well that gets to the "will the real values for E for balsa please stand up?" discussion. The paper I put a link to seems to be the most complete/documented data set out there. It will be interesting to see how designs using these play out in real building.
And then I. Without attempting to get into details - research time for anyone interested in how it is calculated - its about how much material how far from the axis, Simple for solid sticks. What's important to understand is that for the same weight, a hollow stick is stronger in column behavior than a solid stick; moving the material out to the edges- a hollow box beam increases I. Making it denser (to keep same weight) increases E. With the values of E & I increased, E x I goes up quite a bit, stronger column. Box beam does this; ideally, circular tube is theoretically best. The problem is how to build- nobody has cracked that- or at least not said anything about doing so

[jander14indoor]

<SNIP discussion on material vs form stiffness> Box beam does this; ideally, circular tube is theoretically best. The problem is how to build- nobody has cracked that- or at least not said anything about doing so

Not sure if I'm understanding you, but indoor model flyers routinely build circular balsa tubes. Pretty standard practice for outdoor models too. See if the following addresses your needs:

http://www.parmodels.com/Techniques_and ... 0Stick.pdf

You can scale the basic process up or down as needed. I've seen it done with 1/4 inch diameter 1/100 inch wall for crazy light F1D models, done it at the scale in the reference, and scaled it up successfully to 1 inch diameter by 1/32 or 1/20 thick wall. Note, you do have to be careful with wood selections, some grain orientations just don't seem to like being rolled as much as others. Oh, and the paper is important, has to have wet strength, tension on the outer surface while rolling wet really helps prevent cracking until the balsa has dried.

Hope you find that useful,

Jeff Anderson
Livonia, MI

[Balsa Man]

<SNIP discussion on material vs form stiffness> Box beam does this; ideally, circular tube is theoretically best. The problem is how to build- nobody has cracked that- or at least not said anything about doing so

Not sure if I'm understanding you, but indoor model flyers routinely build circular balsa tubes. Pretty standard practice for outdoor models too. See if the following addresses your needs:

http://www.parmodels.com/Techniques_and ... 0Stick.pdf

You can scale the basic process up or down as needed. I've seen it done with 1/4 inch diameter 1/100 inch wall for crazy light F1D models, done it at the scale in the reference, and scaled it up successfully to 1 inch diameter by 1/32 or 1/20 thick wall. Note, you do have to be careful with wood selections, some grain orientations just don't seem to like being rolled as much as others. Oh, and the paper is important, has to have wet strength, tension on the outer surface while rolling wet really helps prevent cracking until the balsa has dried.

Hope you find that useful,

Jeff Anderson
Livonia, MI

Ah, I thought you might have some useful/interesting input on this. Thanks. Given the math for I of a tube (and the weight for column strength compared to square/rectangular (hollow) cross section), this is certainly an avenue for ...the seriously competitive to be exploring.

Hadn't seen the one you linked, had seen similar. Nice clear explanation. The big difference in motor sticks and boom arms is, of course, the magnitude of the axial load (and to some extent the length- and the inverse square relationship of FE to L in Euler's equation). 40kg is, I would think, a whole lot more than the load a twisted rubber band is going to put on. When I said nobody's cracked "that", I was thinking at the kinds of load we're playing with in a boom arm. At these loads, the precision with which its built becomes very important; among other things, localized, aka "Brazier", buckling comes into play. Grain pattern, as you note is/will be a key variable, both in terms of 'will it roll w/o cracking', and what E it actually provides.

Speaking of E for balsa, curious if you'd run across that study that I linked before? I've enjoyed your insights from .....the high end of the flying world over the years, and I'd think this sort of data might be out there in that world- certainly could be of use.

I'm also curious, when you mention 1/32nd,1/20th wall at ~1 inch, what sort of application was that for? At a reasonable balsa density, that configuration provides a column strength that is .....a whole lot more than the force of a twisted rubber band

[jander14indoor]

Well, don't underestimate what a rubber band can do!! Indoor flyers tend to design tubes to operate on the edge of failure to keep flying weight low. We also tend to operate in the range where there is noticable bending short of buckling failure, but have to control that bending to control trim, making us VERY aware of balsa stiffness.

As to the larger tube, it was for a larger plane for outdoor free flight, mine was a P30 with a multi strand 10 gram rubber motor. Its been used for larger models withstanding 35 to 40 gm of rubber. Part of the reason for the size is the motor is inside the tube, needs room to unwind. Another reason for strength is to withstand blown motors. Finally, these classes have fairly large minimum weights (compared to indoors), so there isn't as much drive to keep weights to the absolute minimum.

If it wasn't that document I'd seen something similar.

But due to the wide variability in stiffness for the same density I don't tend to use it much. Instead, indoor flyers tend to test each stick used for stiffness and the holy grail is unusually stiff low density wood. When we want to get quantitative we start calculating stiffness coefficient using a little program like the stiffness calculator on this site: http://www.indoorduration.com/indoordur ... lities.htm

Hmm, now where was the explanation on that... I know it uses euler buckling to calculate stiffness, ratios it to density to get specific stiffness, but it also normalizes it somehow...

Woot, here's an on-line version of same thing showing formula and some explanation: http://www.indoornews.com/custom/utilit ... s_calc.php

Now, the original article...
Aha! Found it. Love the internet. Open INAV issue 103 on this page: http://www.indoornews.com/index.php?opt ... &Itemid=58 relevant article starts on page 22 or so.

Just a comment ong the wide variability of stiffness for the same density. If you ignore this variability, you are ignoring one of the big variables that make two seeming identical structures behave very differently. I suspect the experienced coaches/teams already know this at least intuitively. Here's a way to quantify and test for it and an introduction for advancing students.

Jeff Anderson
Livonia, MI

[Balsa Man]

Just a comment ong the wide variability of stiffness for the same density. If you ignore this variability, you are ignoring one of the big variables that make two seeming identical structures behave very differently. I suspect the experienced coaches/teams already know this at least intuitively. Here's a way to quantify and test for it and an introduction for advancing students.

Jeff Anderson
Livonia, MI

Thanks for adding the great info on this important tool/technique to the knowledge pool that's coming together here. It's that important next step beyond understanding the.....general correlation of density and stiffness; quantifying the variability; getting those few sticks, out of a bunch that weigh the same, that you want. For those that haven't see the SC concept, very much worth understanding and using. One of the interesting things when you look at E vs. density data, is that as density increases, so soes the variability. it's significant in the 4-5-6 lb/cf stuff the flyers use mostly. When you get into higher densities for higher loads, ignoring this variabilty.....gets you in even more trouble.

[retired1]

After dozens of versions and changes, my computer model weighs 5.55g plus the mounting block wt (total 6.+ g) and holds 16kg. It is actually much simpler and easier to build than earlier trials. Will start the students on it this week.

Big error--I forgot to convert 1/16 to cm ( the program was set for metric) and thus the weight was off. A correct weight is 6.62 grams plus the mounting block. Add to that the weight of glue.
May add 1/32 sq x braces to the bottom which will raise the weight some more. Need to build one of each to see if it is necessary.
I was out when they met, so building will start next week. Teacher day this week.

[flyingwatermelon]

I've been using the same computer modeling program for the Tower event last year and I found it to be somewhat accurate. I do have a pressing issue this year though. I've tried to test the model boomilever the way it is supposed to in competition but I can't find a suitable way to "anchor" the boomilever on the side of a "wall". As a result, I oriented the boomilever sideways (like an upside down V) and applied the force perpendicular to the top to generate the same force. However, I'm not sure if this change in orientation will affect the results.

Does anyone know how to anchor the boomilever on the side of a "wall" for the Model Smart 3D program?

[retired1]

Take and place a universal joint on the bottom joints and then "hide" them. (View portion, select supports and hide,and click on the joint)
With them out of the road, do the same thing with the top supports.

This is done with the boom horizontal. I used a "0" X location for the wall.

Without the wall joints anchored, it will not test the boom. ( I found that out the hard way when I copied from HS to MS and forgot to anchor.)